Novel area lighting elements may be realized on the basis of organic light-emitting diodes. As flat lighting device, which have moderate luminance as compared to conventional LEDs (light-emitting diodes), OLEDs are ideally suited for producing flat diffuse light sources (e.g. lighting panels). These light sources will be applicable in as manifold a manner as the displays which are based on organic light-emitting diodes. Due to the thin-film technology used, it will further become possible in the production of OLEDs to realize flexible lighting devices which will then enable new applications in the illumination of rooms, for example.
Since OLEDs represent current-operated components, an important point in the production of large-area lighting elements is homogeneous current density distribution over large areas. With OLEDs, typically at least one transparent contact is formed, which may comprise, for example, a conductive oxide (TCO, transparent conductive oxide). Alternatively, the transparent contact may also be realized as a transparent metal layer. Due to the low conductivity of the TCO layer, said layers frequently limit the homogeneity of the current density distribution as well as the maximally achievable size of a lighting area.
The standard design of a conventional OLED may be summarized as follows. Indium tin oxide (ITO) having a layer thickness of, e.g., about 100 nm may be used as the transparent conductive oxide, for example, wherein the ITO layer frequently is applied to a glass substrate and may serve as an anode. Subsequently, an organic layer or an organic layer structure, which in some cases may comprise up to seven sub-layers or layers, is applied at a layer thickness of about 100 to 200 nm. Finally, a metallic cathode, which may comprise aluminum, for example, is deposited at a layer thickness of about 100 to 500 nm. With large-area lighting elements, the high-impedance resistance of the ITO layer results in an inhomogeneity of the supply of current. For example, the high-impedance resistance of the ITO layer may comprise, e.g., a value of about 10 to 20 ohm/square measure. One reason for the inhomogeneity is, for example, that often the contacts of the ITO layer are possible only on the edge of the lighting element. Thus, a maximally achievable size with a uniformly luminous OLED is limited to 50×50 mm2, for example.
In order to achieve larger dimensions, for example metal reinforcements may be introduced into the ITO layer in the form of grids. Said metal grids (so-called busbars) reduce the effective layer resistance in accordance with their packing density, and thus enable relatively large diode areas to be realized.
Due to the non-transparency of these metal grids, however, the effective lighting area is reduced accordingly. For this reason, metal grids are practical, for example, only up for to about 25 percent of the ITO area. A useful improvement would be an increase in the grid metal thickness, for example, which, however, is not practical because of the structuring, or patterning, possibilities and the layer thicknesses of the organic layers. Additionally, an ITO layer reinforced by metal is contacted only on the outer edges, which limits the maximum surface area of a lighting element despite the effective reduction in resistance.
Said outer edges may be connected to a distributor plate via spring contacts or similar electrical contacts, for example. Since the total current for the anode and cathode is supplied or drained off via these contacts, the contact should be at least divided into two. In a conventional standard form, two edges (for example in the western and eastern directions) are connected to the anode, and two edges (for example in the northern and southern directions) are connected to the cathode, said directions here being used as designations for the lateral edges when the lighting side of the (e.g. quadrangular) plate is looked upon in a perpendicular manner. This does not result in optimum connections, since the connection resistances are different for the cathode and the anode. In addition, such a conventional arrangement of the contacts is not protected against polarity inversion, since, e.g., rotation of the plate by 90 degrees leads to polarity inversion of the lighting plate. However, in a modification of this contact configuration, only one contact exists in each case on the western or eastern side of the plate.
Due to the limited conductivity of the TCO layer, it is also frequently useful, with conventional lighting areas, to subdivide large lighting areas into individual areas and to lead the contacts into the distributor plate.
For another standard configuration, all contacts are led to one side of the lighting plate. In order to achieve homogenous light distribution in this configuration, a lateral wide contacting line is useful. However, this in turn reduces the lighting area. In addition, all of these conventional connection configurations result in an inhomogeneous supply of current, which leads to increased current densities in various regions (points or lines), and thus reduces the homogeneity of the lighting panel or lighting area, since regions having an increased supply of current have higher luminance, on the one hand. On the other hand, this results in increased wear and tear at the same time, which consequently adversely affects the robustness of the lighting device.
On the basis of this conventional technology, there is thus a need for a flat lighting device which guarantees contacting which is protected against polarity inversion, and which at the same time improves homogeneity of the supply of current.